Photoelectric conversion device

ABSTRACT

In a photoelectric conversion device including peripheral ICs, the peripheral ICs are in thermal contact with a substrate having photoelectric conversion elements and a chassis, which covers the peripheral ICs and has high thermal conductivity, via a thermal conductive member, so as to eliminate adverse influences of heat produced by the peripheral ICs such as a low S/N ratio.

This is a divisional application of application Ser. No. 09/107,283,filed Jun. 30, 1998, now U.S. Pat. No. 5,965,872, which is a divisionalof Ser. No. 08/803,106 filed Feb. 20, 1997, now U.S. Pat. No. 5,811,790.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photoelectric conversion device and,more particularly, to a photoelectric conversion device which issuitably used in a radiophotographic apparatus such as a medical,digital X-ray imaging apparatus with a large area and high S/Ncharacteristics.

2. Related Background Art

Not only in Japan in which the population of elderly people isincreasing rapidly but also worldwide, improvement of diagnosticefficiency in hospitals and development of medical equipments withhigher precision are strongly demanded. In such situation, an X-rayimaging apparatus using a film (film type apparatus) has been popularlyused.

FIG. 1 is a schematic view showing the arrangement for explaining anexample of a conventional film type X-ray imaging apparatus. In FIG. 1,an X-ray source 901 is arranged above an object 902 to be inspected (tobe examined) such as a human body (patient), and a grid 903 is arrangedbeneath the object 902 to be inspected. The grid 903 is constituted byalternately arranging a substance that absorbs X-rays and a substancethat transmits X-rays so as to increase the resolution. A scintillator(phosphor) 904 absorbs X-rays and emits visible rays. The visible raysemitted by the scintillator 904 are received by a film 905.

Such film type apparatus has the following problems.

Before a doctor acquires an X-ray image of a patient, a film developmentprocess must be performed, resulting in much labor and time.

Sometimes, when a patient moves during X-ray phototaking or when theexposure amount is improper, phototaking must be inevitably redone.These factors impede improving diagnostic efficiency in hospitals.

A clear X-ray image cannot often be obtained depending on thephototaking angle of the affected portion to be phototaken. For thisreason, in order to obtain an X-ray image required for diagnosis, someimages must be taken while changing the phototaking angle. Suchoperation is not preferred especially when the patient is an infant or apregnant woman.

Furthermore, X-ray image films must be preserved after phototaking for acertain period of time in hospitals, and the number of such filmsbecomes very large in hospitals, resulting in poor efficiency in termsof management in such institutions since the films must be put in andout every time a patient comes to a hospital.

When a patient needs to change the hospital he or she normally visits toseek medical attention for some reason, for example, when a patient in aremote place must undergo diagnosis as highly advanced as that he or shecan receive only in a midtown university hospital or must move abroad,X-ray films after exposure and development must be delivered to the nexthospital by some method. Otherwise, the patient must be subjected tophototaking again in the new hospital. These problems are seriousobstacles against establishing a new system of medical practice infuture.

In recent years, in medical industries, demand for "digitization ofX-ray image information" is increasing. If the digitization is attained,X-ray image information can be managed using recording media such asmagneto-optical disks, and a doctor can acquire X-ray image informationat an optimal angle in real time. When communication systems such as afacsimile system, and the like are utilized, X-ray image information canbe sent to hospitals everywhere in the world within a short period oftime. Furthermore, when the obtained digital X-ray image information issubjected to image processing using a computer, diagnosis with higherprecision than in the conventional method can be realized, and all theproblems that the conventional film method has encountered can besolved.

Recently, an X-ray imaging apparatus that uses a CCD solid-state imagingelement in place of a film has also been proposed to meet demand for"digitization of X-ray image information". For this reason, when a CCDsolid-state imaging element is used, fluorescence, i.e., an X-ray image,from the scintillator must be imaged on the CCD light-receiving surfacevia a reduction optical system. This poses a problem of an increase inscale of the X-ray imaging apparatus. On the other hand, since an X-rayimage is formed via a lens, it is generally accepted that the S/N(signal to noise) ratio is reduced by two to three orders upon passingthrough the lens, and this fact is expected to be disadvantageous uponapplying the CCD solid-state imaging element to medical equipment thatrequire high gradation characteristics.

In recent years, upon development of photoelectric conversionsemiconductor thin films represented by hydrogenated amorphous silicon(to be abbreviated as a-Si hereinafter), so-called contact sensors whichare constituted by forming photoelectric conversion elements on alarge-area substrate and can attain reading by an optical system at anequal magnification to an information source have been developedextensively. In particular, since a-Si can be used not only as aphotoelectric conversion material but also a thin film field effecttransistor (to be abbreviated as a TFT hereinafter), photoelectricconversion semiconductors and a semiconductor layer of TFTs can besimultaneously formed on a single substrate. Since the surface area canbe increased so that an image can be read at an equal magnificationwithout using a reduction magnification system, the S/N ratio can behigher than that of the CCD solid-state imaging element. In addition,since the necessity of the reduction optical system can be obviated, asize reduction of the apparatus can be promoted, and such apparatus iseffective for a small medical institution that cannot assure a largespace, a diagnosis vehicle that carries an X-ray imaging apparatus, andthe like. Owing to these merits, X-ray imaging apparatuses using an a-Sisemiconductor thin film have been extensively developed. Morespecifically, an X-ray imaging apparatus in which photoelectricconversion elements and TFTs using the a-Si semiconductor thin filmreplace the film portion 905 in FIG. 1, and which electrically reads anX-ray, image has been developed.

The X-ray dose on human bodies has an upper limit in hospitals althoughit varies depending on the affected portions. In particular, indiagnosing an infant or a pregnant woman, the dose must be reduced asmuch as possible. Therefore, in general, the light-emission amount of ascintillator (phosphor) that absorbs X-rays and converts them intovisible rays, and the charge amount in an a-Si photoelectric conversionelement which receives fluorescence and photoelectrically converts itare small. In order to obtain a clear image from a weak signal, wiringlines must be shortened as much as possible so as to prevent noisecomponents from superposing on analog signal wiring lines extending froma photoelectric conversion panel, and an analog signal must be receivedby a buffer amplifier to decrease the impedance. Furthermore, in orderto eliminate the influence of noise, the analog signal is preferablyA/D-converted in the vicinity of the buffer amplifier to store digitaldata in a memory.

When a digital X-ray imaging apparatus is constituted using aphotoelectric conversion panel on which photoelectric conversionelements having an a-Si semiconductor thin film are arrangedtwo-dimensionally, it is generally accepted that the pixel pitch ispreferably set to be 100 μm or less in terms of resolution. Also, forchest x-rays of a person, it is generally accepted that the effectivepixel area of the photoelectric conversion elements preferably has atleast a size of 400 mm×400 mm. When a photoelectric conversion panelhaving an effective area of 400 mm×400 mm is formed at 100-μm pitch, thenumber of pixels is as large as 16 million. When photoelectricconversion signals from such large number of pixels are processed,buffer amplifier ICs and A/D-conversion ICs must operate at high speed.In particular, when moving images are to be phototaken, higher-speedprocessing is required, and each IC requires large consumption power.When a large volume of digital data are to be transmitted to a remoteplace outside an X-ray phototaking room at high speed, since ahigh-speed type line driver required for removing transmission errorscomprises an IC mainly constituted by bipolar transistors, it requireslarger consumption power as higher-speed specifications are attained,and becomes an insignificant heat generation source.

In recent years, high-speed CMOS-ICs with small consumption power havebeen developed remarkably, and their further advance in future isexpected. However, as far as the versatile ICs are concerned, theperformance of such CMOS-IC cannot compare with that of ICs usingbipolar transistors. As a consequence, ICs mainly constituted byhigh-speed bipolar transistors must be used, and heat produced by ICsthemselves upon increase in consumption power has a serious influence onan X-ray imaging apparatus.

Heat produced by an IC raises the temperatures of a-Si photoelectricconversion elements and TFTs in the X-ray imaging apparatus. In general,dark currents and photocurrents in a-Si photoelectric conversionelements change in correspondence with the temperature rise. Sincechanges in dark current produce temperature differences in thetwo-dimensional array of photoelectric conversion elements, darkcurrents may vary in the plane to impose an adverse influence in theform of fixed pattern noise (FPN). Also, shot noise in the photoelectricconversion elements may impose an adverse influence in the form ofrandom noise (RDN). Furthermore, the temperature unevenness of thephotoelectric conversion elements upon reading may induce in-planeshading in the output. Moreover, so-called KTC noise (K: Boltzmann'sconstant, T: absolute temperature, C: capacity in transfer system) isproduced upon transferring accumulated signal charges from thephotoelectric conversion elements, and may inflict an adverse influence(RDN). The above-mentioned temperature rise of the photoelectricconversion elements and TFT induces a decrease in S/N ratio of the X-rayimaging apparatus and variations in S/N ratio among pixels, thusdeteriorating image quality. In addition, the reliability of theapparatus may be impaired.

However, since all the X-rays are not always converted into visible raysin the phosphor, some scattered or transmitted X-rays are radiated ontothe above-mentioned buffer amplifier, memory, or other digital ICs inthe vicinity of the photoelectric conversion panel. Such X-raysdeteriorate the performance of ICs formed by crystalline Si, and theapparatus may malfunction over a long term of use, thus posing theproblem of reliability. For this reason, in addition to theabove-mentioned problems, it is desired to take a measure againstexposure of unwanted portions to X-ray radiation.

Such problems may be posed not only in the photoelectric conversiondevice used in the X-ray imaging apparatus but also in a large-area,multiple-pixel photoelectric conversion device which can convert lightinformation into electrical information.

Also, similar problems may be posed not only in a photoelectricconversion device for an imaging apparatus which uses radiation such asX-rays as a light source, but also in a photoelectric conversion devicewhich is used in non-destructive inspections to realize high-speedprocessing and a high-resolution, large-area structure.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the problem that ahigh-quality read image cannot be obtained due to a low S/N ratio as aresult of the temperature rise of a photoelectric conversion element(e.g., an element having an a-Si semiconductor layer) and a switchingelement (e.g., a TFT) caused by heat produced by a high-speed ICrequired for processing a huge number of pixels.

It is another object of the present invention to solve the problem oflow reliability caused by temperature rises in the apparatus.

It is still another object of the present invention to realize aphotoelectric conversion device in which ICs are integrally built inperipheral circuits and which can eliminate adverse influences of heatproduced by the ICs in the peripheral circuits.

It is still another object of the present invention to solve theproblems of low performance and malfunctions in long-term use of ICsconsisting of crystalline Si caused by exposure of the ICs to X-rayradiation.

It is still another object of the present invention to provide aphotoelectric conversion device comprising:

photoelectric conversion means for receiving light that carriesinformation and photoelectrically converting the light into anelectrical signal;

an integrated circuit element (IC) for processing the electrical signalconverted by the photoelectric conversion means; and

a housing which accommodates the photoelectric conversion element andthe integrated circuit element,

wherein a thermal conductive member for thermally connecting theintegrated circuit element and the housing is interposed between theintegrated circuit element and the housing.

It is still another object of the present invention to provide aphotoelectric conversion device comprising at least:

a phosphor for absorbing radiation and emitting light;

photoelectric conversion means for receiving the light emitted by thephosphor and photoelectrically converting the light into an electricalsignal;

an IC for processing the electrical signal converted by thephotoelectric conversion means;

a metal chassis for integrally holding the respective members andtransmitting the radiation; and

a member with a high thermal conductivity, which is interposed betweenat least a portion of the IC and the chassis and/or the radiationabsorbing member.

It is still another object of the present invention to provide aphotoelectric conversion device constituted by at least a phosphor forabsorbing radiation and emitting visible rays, a two-dimensional arrayof a plurality of photoelectric conversion elements for receivingfluorescence emitted by the phosphor and photoelectrically convertingthe fluorescence, a switching element for switching signals from thephotoelectric conversion elements, an IC for driving the photoelectricconversion elements and the switching element, an IC for reading thesignals from the photoelectric conversion elements, an IC (A/D, CPU,memory, etc.) for processing the signal from the IC, a line driver ICfor transmitting processing data to a remote place, a radiationabsorbing member interposed between the photoelectric conversionelements and the plurality of types of ICs, and a metal chassis whichmechanically holds these components and does not absorb any radiation,wherein portions of the plurality of types of ICs contact the radiationabsorbing member (lead plate) or the chassis directly or via a membersuch as heat-dissipation silicone-based grease having a high thermalconductivity.

It is still another object of the present invention to provide aphotoelectric conversion device with built-in peripheral ICs, in whichthe peripheral ICs thermally contact a chassis, which has high heatdissipation characteristics, and covers a substrate having photoelectricconversion elements and the peripheral ICs, via a thermal conductivemember, so as to eliminate an adverse influence of heat produced by theperipheral ICs such as problems of a low S/N ratio, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 11 are schematic views for explaining examples of an X-rayimaging apparatus;

FIG. 2 is a plan view for explaining an example of the reading region ofa photoelectric conversion device;

FIG. 3 is a sectional view for explaining an example of the readingregion of the photoelectric conversion device;

FIGS. 4A to 4C are energy band diagrams for explaining an example of theoperation of a photoelectric conversion element;

FIG. 5 is a schematic equivalent circuit diagram of the photoelectricconversion device;

FIGS. 6 and 8 are timing charts for driving the photoelectric conversiondevice;

FIG. 7 is a schematic equivalent circuit diagram for explaining anexample of a photoelectric conversion device having a two-dimensionalreading unit;

FIG. 9 is a plan view showing an example of a photoelectric conversiondevice;

FIGS. 10 and 14 are sectional views for explaining an example of aphotoelectric conversion device; and

FIGS. 12 and 13 are partial sectional perspective views for explainingexamples of a grid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred example of photoelectric conversion elements that can beapplied to a photoelectric conversion device of the present inventionwill be explained below.

FIG. 2 is a plan view showing photoelectric conversion elements 401 andswitching elements 402 for four pixels in a portion of a two-dimensionalphotoelectric conversion device. In FIG. 2, the hatched portionscorrespond to regions which serve as light-receiving surfaces forreceiving, e.g., fluorescence from a scintillator. Each switchingelement 402 transfers a signal charge photoelectrically converted by thecorresponding photoelectric conversion element 401 toward the processingcircuit side, and is controlled by a signal on a control line 708. Eachswitching element 402 is connected to the processing circuit via asignal line 709. Each contact hole 720 connects the corresponding pairof photoelectric conversion element 401 and switching element 402.

FIG. 3 is a sectional view of the photoelectric conversion device inFIG. 2 taken along a line 3--3 in FIG. 2. FIG. 3 depicts a substrate400, a protection layer 410, a first metal thin film 721, a second metalthin film 722, an insulating layer 725, a semiconductor layer 726, andan ohmic contact layer 727. An example of the method of forming thedepicted photoelectric conversion device portion will be explainedbelow.

A chromium (Cr) film having a thickness of about 500 Å is deposited bysputtering or resistive heating on the substrate 400, at least thesurface of which has insulating characteristics, and is patterned byphotolithography to etch unnecessary areas, so as to form a first metalthin film layer 721. The first metal thin film layer 721 serves as thelower electrode of each photoelectric conversion element 401 and thegate electrode of each switching element 402. A 2,000-Å thick insulatinglayer 725 (a-SiN_(x)), a 5,000-Å thick semiconductor layer 726 (a-Si:H,an amorphous material consisting mainly of silicon atoms and doped witha hydrogen atom), and a 500-Å thick ohmic contact layer 727 (n⁺ -layer)are stacked in turn by CVD in an identical vacuum atmosphere. Theselayers respectively correspond to an insulating layer/photoelectricconversion semiconductor layer/hole injection blocking layer of eachphotoelectric conversion element 401, and also correspond to the gateinsulating film/semiconductor layer/ohmic contact layer of eachswitching element 402 (TFT). Also, these layers are used as aninsulating layer for cross portions (730 in FIG. 2) between the firstand second metal thin films 721 and 722. The thicknesses of these layersare not limited to the above-mentioned values, but may be optimallydesigned in correspondence with the voltages, charges to be used in thephotoelectric conversion device, the incident light amount (e.g.,incident fluorescent amount from the scintillator), and the like. Atleast, the a-SiN_(x) (an amorphous material having silicon and nitrogenatoms) layer preferably consists of a material, which can preventpassage of electrons and holes and can sufficiently assure the functionof the gate insulating film of a TFT, and preferably has a thickness of500 Å or more.

After these layers are stacked, areas that serve as contact holes (see720 in FIG. 2) are dry-etched by RIE (Reactive Ion Etching), CDE(Chemical Dry Etching), or the like, and thereafter, an aluminum (Al)film having a thickness of about 10,000 Å is deposited as a second metalthin film 722 by sputtering or resistive heating. Furthermore, thedeposited film is patterned by photolithography to etch unnecessaryareas. The second metal thin film 722 serves as the upper electrode ofeach photoelectric conversion element 401, the source and drainelectrodes of each switching TFT, other wiring lines (interconnects),and the like. When the second metal thin film 722 is formed, the upperand lower thin films are connected via the contact hole portions at thesame time. Furthermore, in order to form a channel portion of each TFT,a portion between the source and drain electrodes is etched by RIE, andthereafter, unnecessary portions of the a-SiN_(x) layer (insulatinglayer), a-Si:H layer (semiconductor layer), and n⁺ -layer (ohmic contactlayer) are etched to isolate the respective elements. In this manner,the photoelectric conversion elements 401, the switching TFTs 402, otherwiring lines (708, 709, 710), and the contact hole portions 720 areformed.

FIG. 3 illustrates the elements for only two pixels, but a large numberof pixels are simultaneously formed on the substrate 400, needless tosay. Finally, for the purpose of improving humidity resistance, theelements and wiring lines are coated with an SiN_(x) passivation film(protection film) 410. As described above, the photoelectric conversionelements, the switching TFTs, and the wiring lines are formed by onlysimultaneously stacking the common first metal thin film, a-SiN_(x)layer, a-Si:H layer, n⁺ -layer, and second metal thin film layer, andetching these layers as needed. Only one injection block layer need onlybe formed in each photoelectric conversion element, and the above layersexcept for the metal thin films can be formed in an identical vacuumatmosphere.

The device operation of the photoelectric conversion element 401 alonewill be explained below.

FIGS. 4A and 4B are schematic energy band diagrams showing the refreshmode and the photoelectric conversion mode of the photoelectricconversion element, and each show the state in the direction ofthickness of the respective layers in FIG. 3. In FIG. 4A, a lowerelectrode 602 (to be referred to as a G electrode hereinafter) consistsof Cr. An insulating layer 607 consists of SiN and blocks passage ofboth electrons and holes. The thickness of the layer 607 is set to be500 Å or more that can prevent movement of electrons and holes due tothe tunnel effect. A photoelectric conversion semiconductor layer 604consists of an intrinsic (i-type) semiconductor layer of hydrogenatedamorphous silicon (a-Si). An injection blocking layer 605 of an n-typea-Si layer doped with phosphorus or the like blocks injection of holesinto the photoelectric conversion semiconductor layer 604. An upperelectrode 606 (to be referred to as a D electrode hereinafter) consistsof Al. In this embodiment, although the D electrode does not perfectlycover the n-layer, the D electrode and the n-layer always have the samepotential to allow free movement of electrons between the D electrodeand the n-layer, and the following description will be made under theassumption of this fact. The photoelectric conversion element of thisembodiment performs two different operations, i.e., the refresh mode andthe photoelectric conversion mode depending on the way of applying thevoltage between the D and G electrodes.

In FIG. 4A that shows the refresh mode, the D electrode is applied witha negative potential with respect to the G electrode, and the holesindicated by full circles in the i-layer 604 flow toward the D electrodein the presence of an electric field. At the same time, the electronsindicated by open circles are injected into the i-layer 604. At thistime, some holes and electrons recombine in the n-layer 605 and thei-layer 604 and disappear. If this state continues for a sufficientlylong period of time, the holes in the i-layer 604 are wiped out from thei-layer 604.

In order to switch this state to the photoelectric conversion mode shownin FIG. 4B, a positive potential is applied to the D electrode withrespect to the G electrode. Upon applying this voltage, the electrons inthe i-layer 604 instantaneously flow to the D electrode. However, sincethe n-layer 605 serves as an injection blocking layer, holes never flowto the i-layer 604. In this state, when light enters the i-layer 604,the light is absorbed to generate electron-hole pairs. These electronsflow to the D electrode, and the holes move inside the i-layer 604 andreach the interface between the i-layer 604 and the insulating layer607. However, since holes cannot move into the insulating layer 607,they stay in the i-layer 604. At this time, since the electrons moveinto the D electrode, and the holes move to the interface with theinsulating layer 607 in the i-layer 604, a current flows from the Gelectrode to maintain an electrically neutral state. Since this currentis proportional to the amount of electron-hole pairs produced by light,it is proportional to the incident light amount.

After the photoelectric conversion mode (FIG. 4B) is maintained for apredetermined period, when the element is switched to the refresh modestate (FIG. 4A), the holes staying in the i-layer 604 flow to the Delectrode, as described above, and a current corresponding to theseholes flows concurrently. The amount of holes corresponds to the totalamount of light that is incident during the photoelectric conversionmode period. At this time, a current that corresponds to the amount ofelectrons injected into the i-layer 604 also flows. However, the amountof electrons is nearly constant, and can be subtracted from the amountof holes to detect the light amount. That is, the photoelectricconversion element of this embodiment can output the amount of incidentlight in real time, and at the same time, can output the total amount oflight that enters the element within a predetermined period.

However, when the photoelectric conversion period is prolonged for somereason or when incident light has a strong illuminance, often no currentflows although light is incident. Such phenomenon occurs for thefollowing reason. That is, as shown in FIG. 4C, a large number of holesstay in the i-layer 604, these holes reduce the electric field in thei-layer 604, and the produced electrons do not flow to the D electrodebut recombine with holes in the i-layer 604. When the incident state oflight changes in this state, a current flows unstably, but when therefresh mode is set again, the holes in the i-layer 604 are wiped out,and a current proportional to light can be obtained in the nextphotoelectric conversion mode.

In the above description, when the holes in the i-layer 604 are wipedout in the refresh mode, it is ideal to wipe out all the holes, but itis effective to wipe out only some holes. In this case, a current equalto that obtained in the above case can be obtained, and no seriousproblem is posed. That is, the state shown in FIG. 4C need only beprevented at the detection timing in the next photoelectric conversionmode, and the potential of the D electrode with respect to the Gelectrode in the refresh mode, the duration of the refresh mode, and thecharacteristics of the injection blocking layer of the n-type layer 605need only be determined.

Furthermore, in the refresh mode, injection of electrons into thei-layer 604 is not a necessary condition, and the potential of the Delectrode with respect to the G electrode is not limited to a negativepotential for the following reason. That is, when a large number ofholes stay in the i-layer 604, even when the potential of the Delectrode with respect to the G electrode is a positive potential, theelectric field in the i-layer acts in a direction to move the holes tothe D electrode. It is not a necessary condition for the characteristicsof the injection blocking layer of the n-layer 605, either, that it becapable of injecting electrons into the i-layer 604.

The operation of one pixel of the photoelectric conversion device in anX-ray imaging apparatus using the above-mentioned photoelectricconversion elements will be explained below with reference to FIGS. 5and 6. FIG. 5 shows an example of an equivalent circuit including aphotoelectric conversion element and a switching TFT for one pixel, andFIG. 6 shows an example of a timing chart showing its operation. Inorder to refresh the photoelectric conversion element 401, a gate Vg(830) and a reset switching element 805 are turned on while a bias powersupply 801 is set to have a given voltage value (Vr). With thisoperation, the D electrode of the photoelectric conversion element 401is refreshed to Vr, and its G electrode is refreshed to a bias V_(BT) ofa reset power supply 807 (Vr<V_(BT)). After this operation, thephotoelectric conversion element is set in a storage state (readingmode). Thereafter, an X-ray source 901 is turned on, and X-raystransmitted through a human body and a grid 903 are irradiated onto ascintillator 904. Fluorescence produced by the scintillator 904 isirradiated onto and photoelectrically converted by the photoelectricconversion element 401. Since the a-SiN_(x) insulating layer and a-Si:Hphotoelectric conversion semiconductor layer that constitute thephotoelectric conversion element are also dielectrics, the photoelectricconversion element also serves as a capacitive element. That is, asignal charge photoelectrically converted by the photoelectricconversion element is stored in the photoelectric conversion element.Thereafter, the gate Vg of the TFT is turned on, and the signal chargein the photoelectric conversion element is transferred to a capacitiveelement 813. The capacitive element 813 is not formed as a specificelement in FIG. 2, and is inevitably formed by the capacitance betweenthe upper and lower electrodes of the TFT, the cross portion 730 betweenthe signal line 709 and the gate line 708, or the like. Of course, theelement 813 may be formed as a specific element in correspondence withthe design intended. The above-mentioned operations are performed by anamorphous device formed on the insulating substrate except for powersupply and gate control of the TFT. Thereafter, the signal charge in thecapacitive element 813 is transferred to a capacitance 820 in aprocessing circuit by a switching element 825, and a signal is outputvia an operational amplifier 821. Thereafter, the capacitance 820 isreset by a switch 822, and the capacitive element 813 is reset by aswitch 805, thus completing the operation for one pixel.

An example of the photoelectric conversion operation when thephotoelectric conversion elements shown in FIG. 5 are two-dimensionallyarranged in practice will be described below. FIG. 7 is an equivalentcircuit diagram showing an example of the photoelectric conversiondevice obtained by two-dimensionally arranging the photoelectricconversion elements, and FIG. 8 is a timing chart showing an example ofthe operation of this device.

Referring to FIG. 7, photoelectric conversion elements S11 to S33, thelower electrode side of which is indicated by G and the upper electrodeside of which is indicated by D, are connected to switching TFTs T11 toT33. A reading power supply Vs and a refresh power supply Vr arerespectively connected to the D electrodes of all the photoelectricconversion elements S11 to S33 via switches SWs and SWr. The switch SWsis connected to a refresh control circuit RF via an inverter, and theswitch SWr is directly connected to the circuit RF. The refresh controlcircuit RF controls these switches so that the switch SWr is turned onduring the refresh period and the switch SWs is turned on during otherperiods. One pixel is constituted by one photoelectric conversionelement and one switching TFT, and its signal output is connected to adetection integrated circuit IC via a signal line SIG. In thisphotoelectric conversion device, a total of nine pixels are divided intothree blocks. The outputs from three pixels per block are simultaneouslytransferred, and are sequentially converted into outputs by thedetection integrated circuit IC via the signal lines SIG, thus obtainingoutputs (Vout). Three pixels in one block are arranged in the horizontaldirection, and the three blocks are arranged in the vertical direction,thus arranging pixels two-dimensionally.

Initially, shift registers SR1 and SR2 apply Hi-level pulses to controllines g1 to g3 and signal lines s1 to s3. Then, the transfer switchingTFTs T11 to T33 are electrically connected to switches M1 to M3, and theG electrodes of all the photoelectric conversion elements S11 to S33 areset at the GND potential (since the input terminal of an integrationdetector Amp is designed to have the GND potential). At the same time,the refresh control circuit RF outputs a Hi-level pulse to turn on theswitch SWr, and the D electrodes of all the photoelectric conversionelements S11 to S33 are set to be a positive potential by the refreshpower supply Vr. Then, all the photoelectric conversion elements S11 toS33 are set in the refresh mode and are refreshed. At the next timing,the refresh control circuit RF outputs a Lo-level pulse to turn on theswitch SWs, and the D electrodes of all the photoelectric conversionelements S11 to S33 are set to be a positive potential by the readingpower supply Vs. All the photoelectric conversion elements S11 to S33are then set in the photoelectric conversion mode. In this state, theshift registers SR1 and SR2 apply Lo-level pulses to the control linesg1 to g3 and signal lines s1 to s3. In response to these pulses, theswitches M1 to M3 of the transfer switching TFTs T11 to T33 are turnedoff, and the photoelectric conversion elements S11 to S33 holdpotentials therein although the G electrodes of all the photoelectricconversion elements S11 to S33 are open in DC term since they also serveas capacitors. At this time, since no X-rays are incident, no lightenters the photoelectric conversion elements S11 to S33, and nophotocurrent flows. In this state, when X-ray pulses are output, andpass through the scintillator, fluorescence from the scintillatorbecomes incident on the photoelectric conversion elements S11 to S33.This fluorescence contains information concerning the internal structureof a human body. Photocurrents that flow in response to the incidentlight are stored as charges in the photoelectric conversion elements,and the charges are held after the end of incidence of X-rays. Then,when the shift register SR1 applies a Hi-level control pulse to thecontrol line g1, and the shift register SR2 applies control pulses tothe signal lines s1 to s3, outputs v1 to v3 are sequentially output viathe transfer switching TFTs T11 to T13 and the switches M1 to M3.Similarly, other optical signals are sequentially output under thecontrol of the shift registers SR1 and SR2. With these signals,two-dimensional information of the internal structure of, e.g., a humanbody are obtained as the outputs v1 to v9. A still image is obtained bythe operations described so far. However, when a moving image is to beobtained, the operations described so far are repeated.

FIG. 9 shows the packaging state of a detector having 2,000×2,000pixels. To obtain such detector with 2,000×2,000 pixels, the number ofelements bounded in broken lines in FIG. 7 can be increasedtwo-dimensionally. In this case, 2,000 control lines g1 to g2000, and2,000 signal lines sig1 to sig2000 are required. Also, the shiftregister SR1 and the detection integrated circuit IC have a large scalesince they must control and process 2,000 lines. When such elements areimplemented using one-chip elements, one chip becomes very large, and isdisadvantageous in terms of the yield, cost, and the like in themanufacture. In view of this problem, for example, the shift registerSR1 is formed as one chip every 100 stages; 20 chips (SR1-1 to SR1-20)can be used. Also, the detection integrated circuit is formed as onechip for every 100 processing circuits; 20 chips (IC1 to IC20) can beused.

In FIG. 9, 20 chips (SR1-1 to SR1-20) are mounted on the left side (L),20 chips are mounted on the lower side (D), and 100 control lines and100 signal lines per chip are connected by wire bonding. A broken lineportion in FIG. 9 corresponds to that in FIG. 7. Also, externalconnection lines are not shown. Furthermore, the switches SWr and SWs,the power supplies Vr and Vs, the circuit RF, and the like are notshown. The detection integrated circuits IC1 to IC20 generate 20 outputs(Vout). These outputs may be combined into one output via switches, ormay be directly output and subjected to parallel processing.

The preferred embodiments of the present invention using theabove-mentioned photoelectric conversion elements will be described indetail hereinafter with reference to the accompanying drawings.

[First Embodiment]

FIG. 10 is a sectional view of a photoelectric conversion device,according to the first embodiment of the present invention, which issuitably applied as that of an X-ray imaging apparatus. FIG. 11 is aschematic view showing an X-ray imaging apparatus including aphotoelectric conversion device 100 shown in FIG. 10.

Referring to FIG. 11, X-rays from an X-ray source 901 are irradiatedonto an object 902 to be inspected such as a human body or the like, andare subjected to absorption, transmission, and scattering in the objectto be inspected depending on the substances in the body such as thelungs, bones, blood vessels, organs, and the like. The X-rays that havetraversed the object to be inspected travel toward a grid 903.

FIGS. 12 and 13 are sectional views showing the arrangement of the grid.The grid is constituted by alternately arranging a substance (e.g.,lead) that absorbs X-rays, and a substance (e.g., aluminum) thattransmit X-rays. The reason why the grid is arranged is to prevent adecrease in resolution due to X-rays scattered inside the object to beinspected. More specifically, only X-rays in a specific direction (thesectional direction of the grid) pass through the X-ray transmissionsubstance portions (Al) and reach the photoelectric conversion device100. On the other hand, X-rays scattered inside the object to beinspected are absorbed by the absorption substance portions (Pb) of thegrid, and cannot reach the photoelectric conversion device 100.

FIG. 10 is a sectional view showing the internal structure of thephotoelectric conversion device 100 of this embodiment. Referring toFIG. 10, an external chassis 101 serving as a housing of thephotoelectric conversion device consists of a material (e.g., aluminum,a carbon material, or the like) that transmits X-rays, and X-rayscontaining X-ray information in the object to be inspected is irradiatedonto a scintillator (phosphor) 102. The X-rays are excited (absorbed) bythe phosphor in the scintillator, and the scintillator producesfluorescence having a wavelength in the spectral sensitivity wavelengthrange of photoelectric conversion elements 401.

As the material of the scintillator, CsI:Ta, Gd₂ O₂ S:Tb, Y₂ O₂ S:Eu, orthe like is used.

Each photoelectric conversion element 401 photoelectrically convertsfluorescence corresponding to an X-ray image from the scintillator 102,and a signal charge is transferred to a processing IC 403 via aswitching element 402. The photoelectric conversion element 401 and theswitching element 402 are formed on an insulating substrate 400, whichis arranged beneath the scintillator 102 (on the side opposite to theX-ray source). A protection film 410 covers the elements 401 and 402 toprotect them.

As can be seen from FIG. 10, the processing IC 403 is arranged in thevicinity of the photoelectric conversion element 401. This is tominimize adverse influences of external noise that are superposed on thewiring lines if the wiring lines are prolonged to transfer weak signalcharges from the photoelectric conversion elements. The processing IC403 has, e.g., functions of the reset switch element 805, reset powersupply 807, capacitance 820, operational amplifier 821, switch 822, andswitching element 825 in FIG. 5, is or corresponds to IC1 to IC20 inFIG. 9.

The processing IC 403 is mounted on a flexible cable 404. A signal fromthe processing IC 403 is supplied to ICs mounted on a PCB (printedcircuit board; a circuit board manufactured by printing) via a connector408. One of the ICs on the PCB is a high-speed A/D converter, whichconverts an input signal into digital data. After the signal isconverted into digital data, the data is hardly influenced by externalnoise. As other ICs on the PCB, a memory (RAM) for temporarily storingdigital data, a CPU for performing arithmetic processing of data, anonvolatile memory (ROM) for storing a program, a line driver fortransmitting data to a remote place at high speed, and the like aremounted.

These ICs mainly consist of crystalline Si, and their performancesdeteriorate upon irradiation of radiation such as X-rays having veryhigh energy. In the worst case, the functions of these ICs may becompletely lost.

A radiation absorbing member 405 is a shield member for shielding theseICs from radiation, and consists of a material (e.g., Pb) for absorbingX-rays. The member 405 is interposed between the insulating substrate400 on which the photoelectric conversion elements 401 and the TFTs 402are arranged, and the PCB on which the ICs are arranged. With thisarrangement, fluorescence in the visible range from the scintillator 102is irradiated onto the photoelectric conversion elements, and X-rays,which are not converted into visible rays by the scintillator (i.e.,X-rays having traversed the scintillator), pass through the insulatingsubstrate 400 but are absorbed by the Pb plate beneath the substrate400. In this way, such X-rays never reach the ICs on the PCB.

When the Pb plate is arranged, as shown in FIG. 10, high reliability canbe assured in X-ray resistance of the device.

Note that each processing IC 403 is arranged in the vicinity of thephotoelectric conversion element to increase the external noiseresistance (i.e., it is not arranged on the PCB). Since the processingIC 403 also consists of crystalline Si, a Pb member must be arrangedaround it if it is susceptible to X-ray radiation.

FIG. 10 shows an example when each processing IC 403 is covered by aradiation absorbing member such as a lead (Pb) member. If incomingX-rays do not reach the processing ICs 403 arranged around thephotoelectric conversion elements (insulating substrate 400), the-leadmembers 406 can be omitted.

In FIG. 10, high-thermal conductive members 407 represented by, e.g.,silicone-based grease having a high thermal conductivity contact theabove-mentioned processing ICs 403, the ICs on the PCB, and the PCBitself. The thermal conductive members 407 also contact the radiationabsorbing members 405 for absorbing X-rays or the Al chassis 101 thattransmits X-rays.

With this arrangement, heat produced by the ICs mainly constituted bybipolar transistors, which are indispensable for processing informationfrom a very large number of pixels at high speed, can be dissipated tometals such as Pb and Al having a high thermal conductivity. Thesilicone-based grease member that contacts the PCB also contributes toheat dissipation from the PCB.

As the wiring material of the PCB, Cu with a low resistance is normallyused, and Cu is also excellent in thermal conductivity. Hence, thelargest possible Cu solid pattern is formed on a region on the PCB,where ICs are not mounted and a silicone-based heat-dissipation greasemember is arranged thereon, so that the surface, opposite to the PCB, ofthe grease member contacts Pb or Al, thus dissipating heat produced bythe ICs via the PCB.

On the other hand, heat produced by the processing ICs 403 and the ICson the PCB can be dissipated without using any thermal conductivemembers 407 such as silicone-based grease with a high thermalconductivity when they are in direct contact with the Pb plate or the Alchassis. Although the heat dissipation effect becomes slightly lowerthan that obtained when silicone-based grease is used, heat dissipationmay be attained by making these ICs directly contact the Pb plate or Alchassis if heat produced by the ICs is not so large. Of course, inconsideration of reliable thermal conduction, the thermal conductivemembers such as grease members are preferably interposed.

Depending on the distribution of heat produced by the ICs in a singleX-ray imaging apparatus, heat produced by some ICs may be dissipated toPb or Al via the silicone-based grease members and the remaining ICs maydirectly contact Pb or Al to dissipate their heat.

Note that the Al chassis can have a function of mechanically supportingthe above-mentioned phosphor, insulating substrate, IC-mounted PCB, Pbplate, and the like in addition to the function of heat dissipation andthe function of transmitting X-rays.

As the thermal conductive members 407 having a high thermalconductivity, a heat-dissipation adhesion tape having capton or aluminumas a base material may be used in addition to silicone-based grease.

More specifically, as the members 407 having a high thermalconductivity, heat-dissipation members such as heat-dissipation siliconerubber, a heat-dissipation single-sided adhesion tape, aheat-dissipation double-sided adhesion tape, a heat-dissipationadhesive, and the like may be used in addition to silicone-based grease.

As for heat dissipation using the silicone-based grease members, aheat-dissipation structure for mechanically supporting TCPs that packagethe ICs and Pb or Al may be adopted so as to stably fix the ICs to Pb orAl.

In order to improve thermal conductivity, silicone-based grease,heat-dissipation silicone rubber, and a heat-dissipation adhesive mixedwith ceramics-based particles (e.g., aluminum oxide particles) arepreferably used. Also, in order to improve mechanical strength,heat-dissipation silicone rubber mixed with glass cloth is preferablyused.

As the heat-dissipation single-sided adhesion tape and heat-dissipationdouble-sided adhesion tape, an acrylic pressure sensitive adhesive tapecontaining ceramics-based particles (e.g., aluminum oxide particles) isavailable, and both a tape which uses a polyimide-based resin, aluminum,glass cloth, or the like as a base material, and a tape which is formedof an adhesive mass alone without using any base material may be used.

[Second Embodiment]

FIG. 14 is a sectional view of an X-ray imaging apparatus according toanother embodiment of the present invention. The same reference numeralsin FIG. 14 denote the same parts as in FIG. 10. Referring to FIG. 14,corrugations are intentionally formed on the outer surfaces of the Alchassis serving as the housing, and the surface of the Pb plate forshielding X-rays. With this structure, when heat produced by the ICs onthe PCB and the processing ICs 403 is to be dissipated to the radiationabsorbing member such as Pb or the heat-dissipation member such as theAl chassis via thermal conductive members consisting of, e.g.,silicone-based grease, heat-dissipation silicon rubber, heat-dissipationsingle-sided adhesion tape, heat-dissipation double-sided adhesion tape,heat-dissipation adhesive, or the like, the contact area between the Pband Al members and the air therearound increases, thus improving theheat-dissipation efficiency.

As for heat dissipation using the silicone-based grease members, aheat-dissipation structure for mechanically supporting TCPs that packagethe ICs and Pb or Al may be adopted so as to stably fix the ICs to Pb orAl.

The thermal conductive members in this embodiment can also beappropriately selected from thermal conductive members including varioustypes of thermal conductive members described above.

Of course, as the thermal conductive members, the thermal conductivityand elasticity of metals such as copper, phosphor bronze, and the like,may be used, or such metals may be used in combination of theabove-mentioned thermal conductive members.

If energy radiated from a light source including a radiation source(e.g., an X-ray source) can be directly sensed, the above-mentionedphosphor may be omitted, and another wavelength conversion member otherthan the phosphor may be used, needless to say.

In addition, when neither deterioration nor damage of peripheralcircuits such as ICs upon radiation of high energy such as X-rays needbe considered, the above-mentioned radiation absorbing member such as Pbneed not be arranged.

(Effect)

As described above, according to the present invention, since heatproduced by the ICs is dissipated to the radiation absorbing member suchas a Pb plate or the chassis portion of, e.g., Al, to prevent excessiveheat from being conducted to the photoelectric conversion elements andTFTS, or such heat is reduced to a negligible level if it is conducted,the problem of a low S/N ratio can be solved, and the reliability of thephotoelectric conversion device and a system using the same can befurther improved.

Also, according to the present invention, in the photoelectricconversion device having the ICs as peripheral circuits, the problemcaused by adverse influences of heat produced by the ICs can be avoidedor substantially avoided.

In addition, according to the present invention, since a high S/N ratiocan be assured without increasing fixed pattern noise or random noise,high-quality read images can be provided for a long period of time orsuccessively, and over a long term.

Furthermore, according to the present invention, the high S/N ratio ofthe photoelectric conversion element using an a-Si semiconductor thinfilm can be assured without increasing fixed pattern noise or randomnoise, and high-quality read images can be provided.

Since the photoelectric conversion device of the present invention canbe one factor that realizes a digital X-ray diagnostic system includingan X-ray imaging apparatus with a photoelectric conversion device, itcan contribute to improvement in diagnostic efficiency in hospitals andin building inspections, and allows to build a world-wide diagnosticinformation network system in future.

Moreover, according to the present invention, "digitization of X-rayimage information" strongly demanded in the medical industries in recentyears can be realized, and not only the diagnostic efficiency inhospitals can be greatly improved but also a nation-wide medicaldiagnostic information network can be built. Accordingly, the diagnosticefficiency in the whole medical field can be improved. For example, apatient even in a remote place can undergo diagnosis as highly advancedas that he or she can receive only in a midtown university hospital.

What is claimed is:
 1. A photoelectric conversion device comprising:asensor substrate having a sensor formed on a surface thereof and havinga metal plate provided on a side thereof opposite to the sensor-formedsurface side; an IC provided on the metal plate via a thermal conductivemember; and a chassis housing the sensor substrate, sensor, metal plateand IC therein.
 2. The device according to claim 1, wherein the metalplate has an unevenness formed on a surface thereof.
 3. The deviceaccording to claim 2, wherein the unevenness is formed on the side ofthe metal plate on which the IC is provided via the thermal conductivemember.
 4. A photoelectric conversion device comprising:a sensorsubstrate having a sensor formed thereon; an IC for processing a signalfrom the sensor; and a chassis housing the sensor substrate and the ICtherein, wherein the IC is fixed via a thermal conductive member to thechassis, and wherein an unevenness for heat radiation is formed on asurface of the chassis.